The present invention relates to an apparatus for treating fiber suspension. The apparatus in accordance with the present invention is especially applicable for pulp screening in the wood processing industry and especially for the separation of light particles from the fiber suspensions. Moreover, the invention relates to a rotor structure of the screener.
There are in principle two known rotor types, which both are in common use and the purpose of which, as known, is to maintain the filter surface clean, in other words to prevent the formation of fiber matting on the filter surface. One of the rotor types may be exemplified by a rotor in accordance with U.S. Pat. No. 4,193,865, in which a rotatable rotor has been arranged inside a cylindrical, stationary filter cylinder and which rotor comprises blades which are positioned close to the surface of the filter cylinder, and which blades in the construction of the example are in an angle position relative to the shaft of the cylinder. The filter surface is subjected to pressure pulses by the moving blades, which thus clear the openings of the surface. There are also embodiments, in which the blades are located on both sides of the filter cylinder. At this point the suspension to be treated is introduced to the inside or the outside of the cylinder and the discharge of the accept is from the outside or the inside of the cylinder, respectively.
The other type is exemplified by a rotor in accordance with U.S. Pat. No. 3,437,204, in which the rotor is substantially a closed cylindrical object, the surface of which is provided with almost hemispherical protrusions. When this type of apparatus is used pulp is supplied to a treatment volume between the rotor cylinder and the filter cylinder outside the rotor cylinder, whereby the purpose of the protrusions from the rotor is both to press the pulp against the filter cylinder and to draw (with the trailing edge) the thickened fiber matting off from the openings of the filter cylinder. Because this kind of a construction has a highly thickening effect on the pulp, three dilution water conduits have been mounted at different levels in the filter cylinder in the structure in accordance with this patent, so as to carry out the screening of fiber suspension satisfactorily. A "bump rotor" is also illustrated in U.S. Pat. No. 3,363,759, in which the rotor is slightly conical.
Moreover, there are other embodiments of the above-mentioned cylindrical rotor, which are illustrated in different publications with different protrusions on the filter cylinder side.
U.S. Pat. No. 4,356,085 discloses a knotter, which has plough-like protrusions of plate material on the surface of the cylindrical rotor cylinder, which are used to bring about strong mixing forces in the pulp between the rotor and the filter cylinder so that the fibers would pass the filter cylinder as effectively as possible and the knots, shives and like would separate.
U.S. Pat. Nos. 4,188,286 and 4,202,761 disclose a filter apparatus, which has a rotatable cylindrical rotor inside the filter cylinder. The rotor surface on the filter cylinder side is provided with protrusions having a wedge-like radial cross-section in such a way that they have a front surface evenly rising from the rotor surface, a surface parallel to the rim of the rotor and extending closest to the filter cylinder and a rear surface substantially perpendicular against the rotor surface. These protrusions are arranged on the surface of the rotor cylinder at a particular angle position relative to the axial direction so that all the protrusions of the rotor are at the same position relative to the axis of the rotor.
When the pulp is supplied to outside of the filter cylinder and the accept is discharged from the inside of the filter cylinder, in other words on the rotor side, the rotational direction of the rotor is such that the angular position of the protrusions subjects the accepts to a downward force component and the inclined/rising surface operates as a front surface. In U.S. Pat. No. 4,188,286 the rotor surface is provided with openings which are located in the flow direction behind the protrusions, and through which accepts pulp which has been screened through the screen is discharged to the accept conduit of the apparatus from between the screen and the rotor. The openings of the rotor surface are thus used for the discharge of accepts, providing an in-flow-type screener.
Experiments have proven that the previously described apparatus embodiments do not operate satisfactorily in all fields of application. For example, the first described blade rotor causes pressure pulses that one too strong on the accepts side of the filter cylinder, so that such a structure cannot be used with the head boxes in paper machines, which should not have fluctuation of pressure in the suspension. The apparatus also tends to dilute the accepts, which is why the blade rotor cannot be applied in places where pulp of uniform consistency is required. Since the number of the blades in the blade rotor is small (4 to 8 blades), fiber matting is always formed on the filter cylinder before the next blade wipes it off. Thus the use of the filter is not efficient. Also, this rotor type is expensive to manufacture due to required exact shapes and careful finishing.
The substantially cylindrical rotor provided with almost hemispherical protrusions, described above, is eminently suited for some applications. However, the head box of a paper machine is not one of them, because the pulp suspension arriving at the head box must have a uniform consistency and size of fibers, and the machine screen should not change these values. This kind of a "bump rotor" tends to dilute the accepts and also causes fluctuation in the consistency. In performed experiments it was discovered that one of the described rotor types diluted the accepts within the range of -0.15 to -0.45%, the desired accept consistency being 3%. A consistency fluctuation, as mentioned, of +/-0.5%, is too much when aiming at a uniform and qualified final product. On the other hand, in a filter including a "bump rotor" fractionation also takes place (in other words, the interrelation of the fractions in the fiber suspension is not the same as that of the originally supplied pulp). With the "bump rotor" the change grade of fractionation varies from 5 to 10% according to the clearance between the filter cylinder and the rotor. A change grade for a blade rotor is about 20%, so a bump rotor is a considerable improvement.
The deficiencies of a filter apparatus provided with a "bump rotor" have led to some attempts for improvement, of which examples are the above-mentioned bringing of the dilution water to the filter surface, and in providing a slight conical form of the rotor.
The method in accordance with U.S. Pat. No. 5,000,842 (the disclosure of which is incorporated by reference herein) and the apparatus developed for realizing it represent the most recent developments. The method in accordance with this patent is characterized in that fiber suspension is subjected to axial forces changing in intensity and effective direction, the direction and intensity of which axial forces are determined on the basis of the axial position between the point of application and the counter surface of the filter cylinder and with which the axial speed profile of fiber suspension is changed while maintaining the flow direction constantly towards the discharge end. The apparatus in accordance with U.S. Pat. No. 5,000,842 is characterized in that at least one of the counter surfaces of the rotor and the filter cylinder towards the other is provided with at least one protrusion or the like, the direction of the front surface of which differs according to the axial direction and which subjects an axial force component to the pulp particle in the space between the counter surfaces, the intensity of which changes as a function of the axial position of the counter surfaces of a pulp particle and which changes the speed profile of the fiber suspension flowing between the counter surfaces.
Although the apparatus and method in accordance with U.S. Pat. No. 5,000,842 are superior to prior art techniques, the method described in the above-mentioned patent and the technical arrangement realizing it have further been developed. Detailed experiments have shown that all rotors using any kinds of protrusions, whether blade-like, hemispherical, rectangular or any other shape, have pulp from the top of the protrusion towards the trailing direction, the consistency and reject content of the pulp being higher than the average in the screening area. This, of course, results from the fact that the pulp has been subjected by the protrusion to a pressure stroke thus pressing acceptable material from the pulp through the filter surface, whereby both liquid and acceptable fibrous material flows through the screen surface. The experiments have also shown that thicker pulp which contains more rejects is liable to remain against the screen surface, although the effect of the rotor protrusion in the pulp portion ceases. This, of course, lowers the capacity of the screener, because fresh or less screened pulp must first pass through the pulp layer of higher consistency in order to pass through the filter. When a thickener is used the fiber matting accumulated on the filter surface requires that the filtrate must not only pass through the openings of the filter surface, but also must be pressed through the fiber matting.
The problem described above has been eliminated in U.S. application Ser. No. 07/804,534, filed Dec. 11, 1991, which is a continuation of Ser. No. 07/524,752, filed May 17, 1990, now abandoned, which has a rotor construction such that the above-mentioned thicker and coarser pulp portion is transferred away from the filter surfaces towards the rotor surface so that the fresher pulp comes into direct contact with the filter surface, whereby the deficiencies of the prior art apparatuses may be eliminated. The apparatus in accordance with this U.S. application is characterized in that the counter member of the filter surface is equipped with at least one guiding plate, which guides the enriched coarser and/or thicker suspension away from the vicinity of the filter surface.
Another embodiment of the apparatus in accordance with the above-mentioned U.S. application is characterized in that the counter surface of the filter surface is provided with at least one member, which is formed by a protrusion formed on the counter surface and a guiding plate extending from the counter surface higher than said protrusion, which protrusion and guiding plate leave between them an opening, through which the thicker and/or coarser fraction may flow under the guiding plate.
When performing test drives with these devices, it became apparent that light reject had accumulated under the guiding plate located in the flow direction behind the protrusion and descending towards the surface of the counter plate, in other words they were plastics particles, which had practically speaking filled the entire space between the guiding plate and the counter plate. When this was studied more closely it was seen that an area of reduced pressure was generated behind to the discharge side of the protrusion and especially under the guiding plate, where light particles easily accumulate while the centrifugal force moves heavier particles, such as knots and shives, etc. towards the screen surface. Thus by providing the discharge side of the protrusion or the guiding plate with openings, it is possible to remove the light reject directly to the inside of the rotor, from where they may be discharged according to the technique known from U.S. Pat. No. 4,634,521. Prior to the present invention the light rejects have accumulated on the rotor surface and drifted with the pulp flow to the lower end of the rotor, from where they have been able to turn around the periphery of the rotor to the inside of the rotor, and from where it is easy to be discharged with an apparatus according to U.S. Pat. No. 4,634,521 (the disclosure of which is incorporated by reference herein).
The apparatus in accordance with the present invention is characterized in that at least one of the protrusions on the rotor surface is provided with an opening for guiding the light reject through the rotor surface, and is seen from the following specifications and claims.